This invention relates to an optical element such as a lens, an optical element holding structure for holding the optical element inside a lens barrel, and to an optical-lens barrel and an optical communication module.
Plastic lens are often used in optical devices in view of the fact that they have the advantage of light weight and low cost. However, if temperature changes occur due to the environment in which the device is used, internal stress is generated inside the plastic lens that is supported inside the lens barrel, and the internal refractive index changes due to this stress and causes the double refraction phenomenon to occur. The internal refractive index changes can be expressed using the following formula, but the photo-elasticity index for resin is dependent on temperature as shown in FIG. 9, and tends to increase as the temperature increases, and thus the internal refractive index changes to a great extent due to temperature increases.Internal refractive index=Amount of change in stress×photo-elasticity index
Patent Document 1 which is listed below discloses a lens holding structure for fixing the lens by pressing it with a ring having screws. In the case of a glass lens, stress is not generated by pressing with the ring, but in the case of a plastic lens, stress is concentrated at the area where the ring is pressed, and as described above double refraction occurs at the lens.
Patent Documents 2 and 3 which are listed below, each disclose a lens mechanism in which stress is applied to the outer periphery of the lens and the refractive index is changed, an imaging device that uses said lens mechanism, a lens for varying its focal length and a method for varying the focal length. However, when comparing the change in stress in the optical axis direction and that on the surface perpendicular to the optical axis, the change in stress in the optical axis direction is significantly different from the change in stress on the surface perpendicular to the optical axis, causing internal double refraction.
Patent Document 4 which is listed below, discloses an optical unit which uses a plastic lens having a structure in which an external force is applied to the plastic lens and the applied external force is caused to operate in the direction where the internal stress of the plastic lens is reduced, and then stress is reduced after releasing the internal stress and adjusting the optical characteristics, as well as a twist and release method and a mounting method for the plastic lens. In this optical unit, a mechanism is provided for reducing the stress on the mounting portion of the plastic lens, but this causes the optical axis to shift. In particular, when the lens undergoes repeated thermal expansion and thermal contraction due to temperature changes the optical axis shifts.
A conventional example of the optical communication signal transmission module that is disposed at the terminal for sending and receiving optical signals in the optical communication system formed by the optical transmission paths such as those of optical fiber is shown in FIG. 13. In a conventional optical communication signal transmission module 100 shown in FIG. 13, the optical signals from the optical transmission paths of the optical communication system are radiated from the end surface of optical fiber 101 and transmitted through a wavelength splitting filter 102 and then passed through a collimator 106 and then received at a light receiving element 103. In addition, the optical signals from a light emitting diode 104 passes through a collimator lens 109 and is reflected at the wavelength splitting filter 102 and entered onto the end surface of the optical fiber 101, and then sent to the optical transmission path of the optical communication system. The collimator lenses 106 and 109 are fixed inside lens barrels 105 and 108 by rings 107 and 110 respectively. The rings 107 and 110 are mounted by YAG laser welding.
In FIG. 13, the lens barrels 105 and 108 and the rings 107 and 110 are formed of metal, and the collimator lens as 106 and 109 are formed of glass. In this manner, because of the difference in the linear expansion coefficient, when the plastic lens 106 and 109 are fixed to the metal lens barrels 105 and 108, distortion occurs between the lens 106 and 109 and lens barrels 105 and 109 due to temperature changes caused by the environment in which the optical communication transmission module 100 is used. It is to be noted that here the linear expansion coefficient of the plastic is 6.00×10−5 for example, the linear expansion coefficient of the metal is for example, 1.08×10−5.
Stress inside the plastic lens is generated due to the above-described distortion, and furthermore the middle position of the lens sometimes shifts due to the method for fixing the lens inside the lens barrel. This shift in the middle position of the lens causes a relative shift of the middle position on the surface of the end of the optical fiber with respect to the light receiving element and the light emitting diode. For example, as shown in FIG. 14, if the x-axis position on the surface of the end surface of the optical fiber shifts from the center by only a few μm (same for the y-axis position), the output value rapidly decreases. In this manner, the signal-sending and signal-receiving properties of the transmission module are deteriorated by the environment in which the optical communication transmission module is used.
[Patent Document 1] Japanese Patent Application Laid-Open No. 6-94957 Publication
[Patent Document 2] Japanese Patent Application Laid-Open No. 7-159692 Publication
[Patent Document 3] Japanese Patent Application Laid-Open No. 9-49905 Publication
[Patent Document 4] Japanese Patent Application Laid-Open No. 10-186197 Publication